The invention concerns a mold, particularly a mold for press forming doses of plastic material being separated from an extruder.
Specifically, but not exclusively, the invention can be applied to form plastic caps, for example to close containers.
The prior art includes the mold of FIGS. 1 and 2 which are suitable for compression molding plastic material doses. This mold of a known type includes a first half-mold 1 (lower matrix) and a second half-mold 2 (upper punch) being axially movable towards one each other in order to assume at least one open position (not shown) and at least one non-end closed position (FIG. 1). A tubular element 3 being axially slidable is arranged around the second half-mold 2. In the non-end closed position (closed and filled mold, namely filled with at least one dose), the lower end of the tubular element 3 contacts an axial abutment 4 arranged on the first half-mold 1, the bottom of the forming cavity has a thickness equal to D, and the axially movable actuator element 5 which bears the first half-mold 1 is placed at an axial distance equal to E from an abutment of end stop 6 being arranged on the press body 7. In an end closed position (FIG. 2, closed and empty mold, namely without a dose), the above-mentioned axially movable actuator element 5 has contacted the abutment of end stop 6 and the bottom thickness of the forming cavity has been reduced to a value T2 equal to D−E. The bottom of the forming cavity is defined above by the lower surface of the second half-mold 2, which is placed at a fixed distance equal to C3 from the abutment of end stop 6. It should be noted that the precision of the value T2 of the thickness dimension of the bottom of the forming cavity, without a plastic material dose, also depends on the precision of the above-mentioned distance C3.
One of the drawbacks of the known mold described above can be seen when the bottom thickness D of the object to be formed is very thin.
In this case, the nominal thickness T2 of the forming cavity in the end closed position (closed and empty mold) could be significantly reduced, for example being of the same order of magnitude of the dimensional tolerances (particularly being bound to the different elastic and/or thermal deformations of the different molds which are usually carried by the press and to the elastic and/or thermal deformation itself of the press complex structure). This results in a hard contact risk, or in an excessive distance between the “wet” surfaces of the two reciprocally facing half-molds when the empty mold closes, without plastics, where with wet surfaces it is intended for the forming cavity surfaces to contact the plastics during the forming phase.
This hard contact can result in a collision between the two half-molds during the closure phase, causing considerable damage to the mold.
The risk is increased when the press is formed by a complex apparatus, such as for example a forming carousel which rotates a plurality of molds, because in this case the precision of the distance T2 with an empty mold would inevitably be very poor as it would depend on a long chain of dimensional tolerances going through the forming carousel. The deviation of the real dimension from the nominal dimension T2, generally different from mold to mold, could be excessive for at least one or more molds, leading to the hard contact.
Practically, in the compression molding carousel for doses of plastic material of a known type, it is not possible to mold objects (caps) having a bottom with nominal thickness lower than about 0.2-0.3 millimeters.
U.S. Pat. No. 6,736,628shows in FIG. 1-3 a compression mold which includes a tubular element 57 being axially movable upwards against a spring 74. The compression mold shown by U.S. Pat. No. 6,736,628, when closed empty and without plastics, will have to assume a closing position different from the closing position with plastics.
Generally, in compression molding, it is the reciprocal movement of the mold parts that transmits the deformation force to the plastic dose to be deformed; this deformation force must be transmitted during all the molding phase, also following the shrinking of the material itself during the molding final phase. For this reason the actuator element 5 shown in FIG. 1 will not have to be in abutment (E>0). In the compression molding according to the known art shown in FIG. 2, the closing position without plastics is defined by the abutment 6 and thus the gap T2 is affected by a quite “long” kinematic chain.
Therefore, the compression mold shown by U.S. Pat. No. 6,736,628 could show the risk of hard contact between the two half-molds during the closure phase without plastics, especially when the bottom thickness of the object to be formed is very thin. All this is not dependent on the dimensional tolerances, whose effect adds to the one just mentioned, and to which deformations caused by thermal components can be added.
As it will be shown below, the solution, which is the object of the present invention, firstly allows reducing the tolerance chain, which affects the definition of the closed mold geometry in a known compression mold, and secondly allows making this tolerance chain substantially independent from the press deformation.
In accordance with embodiments of the present invention, as it can be seen for example in FIG. 4, the condition of a closed mold without plastics is determined, particularly, by the closure of the first half-mold 1 on the tubular element 3 (abutment 4) and through this on the end stop 8: the gap T1 is determined by the only dimensional tolerances of the elements shown as 1, 2, 3, and the actuator is not abutting (A2>0). Therefore, the condition of a closed mold with plastics (FIG. 3) causes forces to be transmitted to the plastic (non-abutting actuator, A1>0) with the element 3 in mold closure on the first half-mold 1 (see abutment 4), but not abutting against the second half-mold 2 (B>0).
Moreover, U.S. Pat. No. 6,736,628 shows in FIG. 4 an injection mold which does not have the spring 74 and, since it is not a compression mold, it is outside the context of the present application. It should be noted that, in an injection mold, the plastic is injected after the mold closure. In the injection molding system, the plastic in the fluid state is injected inside the forming cavity when it is already “closed”, while in a compression mold—as the one according to embodiments of the present application—two closed configurations exist, one without plastic and the other with plastic. In the injection mold shown in FIG. 4 of U.S. Pat. No. 6,736,628, the mold is closed without plastic and the collar 60 must necessarily contact the cup-shaped element 82. It cannot be different, since the plastic must be injected in the forming cavity. In the mold of FIG. 4, a closed position without plastic where the forming cavity volume is less than the closed position with plastic would be unacceptable, because in this closed position with plastic—being purely hypothetical and totally unsuitable—the collar 60 and the element 82 will inevitably lose contact from each other, with the devastating effect of the injected plastic coming out from the forming cavity, which therefore will not be able to be considered as effectively closed. This hypothetical detachment between the collar 60 and the element 82, in the version of FIG. 4 of U.S. Pat. No. 6,736,628, will be even more unacceptable because, as it is known, in order to facilitate the escape of air, ventings are often made whose dimensions are such to let the air pass through, but not the melted plastic and the minimal increment of these ventings will be enough to cause the plastic escape due to the detachment of the above-mentioned parts 60 and 82. In embodiments of the present compression mold, as it will be better explained in the following detailed description, the abutment 4 always contacts the half-mold 1 (see FIGS. 3 and 4 of the present application), preventing the plastic escape, both without plastic and with plastic. This happens also in instances where ventings are provided.
Moreover it should be noticed that in case the thickness of one wall (for example the bottom) of the object to be formed is very thin, the flow sections for the fluid plastic are very narrow, making it extremely difficult for the plastic to flow. Consequently, as a practical matter, injection molding is totally unsuitable to form very thin walls, unless extremely fluid plastics are used, resulting in a limitation of the application field.
U.S. Pat. No. 5,786,079 discloses an apparatus for manufacturing screw closures by pressure molding. US 2007/098833 discloses a machine for compression molding closure shells.